I regret a few cards...
1) Diamond Stealth 64.
It was a Trio64-based card, but apparently a REALLY REALLY slow one. Friend of mine had a cheap no-name Trio64 card which was considerably faster.
2) Matrox Mystique 2 MB.
Two sides to this story... For software rendering and video playback I really loved this card... Thing is, it had 3d acceleration aswell, but with only 2 MB it was useless. Stuff either didn't run on it at all, or it had gray patches where the textures were supposed to be.
I should have gone for the 4 MB version, I suppose.
The only thing I ever played with 3d acceleration was Tomb Raider, in 512x384.
3) Videologic Apocalypse 3Dx.
I really liked the card because of the PowerVR technology. But it was let down by poor driver and software support. It didn't support Glide at all, and DirectX support wasn't that good, performance was quite poor. The only thing that worked quite well on it were native PowerSGL games and miniGL.
But there weren't a whole lot of those around. I played only Tomb Raider and GLQuake/Quake II on the card. So it was pretty useless really. Then again, it was very cheap by the time I bought it, and I bought it for an old computer anyway, so it's no big deal.
4) Matrox G450.
Having been a Matrox fan for years, I pretty much 'missed out' on the revolution that nVidia had started. I bought the G450 for my brothers PC to replace a G200 card that had died.
Thing is, the G200 was one of the fastest and most feature-rich cards when we bought it... but in between nVidia took over, and I hadn't quite realized just how much better the nVidia cards really were. When I bought the G450, the GeForce 2 had just been launched, I believe (or maybe it was the GeForce DDR, anyway, one of the early GeForces).
Later I bought a GeForce2 GTS for my own PC, and it was a real eye-opener. When I saw the raw power of this card I thought "Shit, I saw GeForce cards in the store when I went and bought the G450, but I just ignored them because I didn't know about them". Those were the cards I should have bought instead of the G450.
5) Hercules Prophet Kyro II.
Again, the charm of a PowerVR GPU... but it was let down by the drivers and the fact that the card didn't use a conventional Z-buffer... Lots of D3D software wasn't written properly, and as such had depth-sorting issues on Kyro cards. When it worked, the Kyro II was really fast, especially considering its low price and modest specs (didn't even have DDR memory). But it didn't work very often, so I returned it and got a different card instead... a Radeon 8500. The only card I ever returned actually.
Also, I'm surprised to see so many people mention the 9600Pro/XT cards. I had a 9600XT 256, with an Athlon XP1800+ system running on a VIA KT133A chipset, and an SB Live! card. Recipe for disaster, one would think... but after a few driver and BIOS updates, the system was perfectly stable, and I've used it throughout the Doom3/Far Cry/HalfLife 2 era, where the card performed admirably. One of the best cards I've had, and one of the longest spans before upgrading to a new card. In fact, I upgraded the card because I bought a new motherboard with PCI-e, and the card was AGP. Otherwise I'd have kept it even longer. Instead, I bought a cheap 7600GT to make my new system functional and hold me over until the DX10 cards arrived. I then upgraded to an 8800GTS320, which I still have.